1. Field of the Invention
The present invention relates to a reception device and a communication control method in a mobile communication system, and more specifically to a reception device used as a downlink mobile station which effects transmission diversity control in the mobile communication system and a communication control method for use with a mobile communication system including the reception device.
2. Description of the Related Art
Generally, “fading” occurs in wireless communication, and the fading considerably degrades the transmission quality, that is, a bit error rate characteristic.
A method for compensating for the degradation of the transmission quality due to the fading can be a commonly known “transmission diversity” (for example, “3GPP TR25.214 v5.8.0”, March 2004; hereinafter referred to as non-patent document 1). Described below is the “closed loop transmission diversity mode 1” as a type of transmission diversity.
FIG. 10 shows the configuration of the transmission unit for realizing closed loop transmission diversity, and FIG. 11 shows the configuration of its reception unit.
By referring to FIG. 10, the transmission unit includes: a channel coder 210 for inputting a transmission data sequence; weight units 211 and 212; an antenna weight generator 215 for assigning a weight to the weight units 211 and 212 according to feedback information from a mobile station; spreading units 213 and 214; a spreading code generator 216 for assigning a spreading code to the spreading units 213 and 214; and multiplex units 217 and 218 for transmitting transmission signals to an antenna provided corresponding to antennas 1 and 2.
By referring to FIG. 11, the reception unit includes: a CPICH despreading unit 310 for despreading a CPICH (common pilot channel) using a predetermined scrambling code and a channelization code of the CPICH in response to input received signals; a phase comparison unit 320 for determining a phase difference between signals from a first transmission antenna and signals from a second transmission antenna using received CPICH symbols; and an FBI bit generation unit 360 for receiving a determination result about a phase difference and generating an FBI (feedback information) bit.
The reception unit is also constituted by including: a DPCH despreading unit 311 for despreading a DPCH (dedicated physical channel) using a predetermined scrambling code and a channelization code of the DPCH on input received signals; an antenna verification unit 321 for estimating the phase of signals from two antennas using the CPICH symbols received from the CPICH despreading unit 310 and dedicated pilot symbols received from the DPCH despreading unit 311 (hereinafter referred to as antenna verification); a channel estimate unit 330 of a first transmission antenna for obtaining channel estimated values of signals from the first transmission antenna using the CPICH symbols; and a channel estimate unit 331 of a second transmission antenna for obtaining a channel estimated values of signals from the second transmission antenna using the CPICH symbols.
Furthermore, the reception unit is constituted by including: an RAKE combining unit 340 of the first transmission antenna for performing RAKE combining on DPCH symbols from the first transmission antenna; an RAKE combining unit 341 of the second transmission antenna for performing RAKE combining on DPCH symbols from the second transmission antenna; and a DCH (data channel) channel decode unit 350 for combining the DPCH symbols from the first transmission antenna with the DPCH symbols from the second transmission antenna, and then decoding a channel.
In the closed loop transmission diversity realized by the above-mentioned transmission unit and reception unit, two transmission data sequences are respectively multiplied by W1=A1eiφ1 and W2=A2eiφ2 based on the feedback information (FBI) bit from the mobile station, and then spreading is performed, and the data sequences are transmitted.
First, a CPICH as a common pilot channel is transmitted from the two antennas with the same carrier phase. The CPICHs transmitted from the two antennas are spread using the same spreading code and pilot symbols are changed, thereby realizing orthogonalization. The reception device generates an FBI bit for control of a reception carrier phase difference depending on the reception carrier phase difference of the signals demultiplexed after the despreading of the CPICHs from the two antennas such that the signal sequences transmitted from the two antennas can be in phase at the reception terminal of the mobile station, and transmits it over the DPCCH of the dedicated physical channel DPCH in the uplink from the mobile station to the radio base station. Thus, by controlling the transmission carrier phase of the antenna 2 using the FBI bit from the mobile station, a bit error caused by a drop of the received signal power due to the fading can be reduced. The transmission unit of the base station multiplies the transmission data sequences of the two antennas by the transmission antenna weights W1 and W2 generated based on the FBI bit from the mobile station, and transmits the transmission data sequences multiplied by the transmission antenna weights from each antenna.
In the closed loop transmission diversity mode 1 specified in the 3GPP (3rd Generation Partnership Project), the transmission carrier phase of the dedicated physical channel DPCH of the second antenna is controlled with the resolution of the carrier phase of π/4 such that the received signals from the two antennas can be substantially in phase when received by the mobile station. Described below in more detail is the operation performed when the closed loop transmission diversity mode 1 is applied to the dedicated physical channel DPCH.
The transmission amplitudes of the two antennas in the slot n are A1,n=A2,n=1/√2, and the transmission carrier phases are φ1, n=0, φ2, n={π/4, ±3π/4}.
The mobile station estimates the reception carrier phases θCP1,n and θCP2,n of the CPICHs transmitted from the two antennas, and generates an FBI bit bn at the slot n.
That is, at the even slot n,                if −π/2≦(θCP1,n, θCP2,n)≦π/2 then bn=0,        otherwise bn=1        
At the odd slot n,                if 0≦(θCP1,n, θCP2,n)≦π then bn=0,        otherwise bn=1        
The base station determines the provisional transmission carrier phase ψ2,(n+1) at the (n+1) slot of the DPCH at the second antenna as follows depending on the decode result bn′ (when there is no FBI bit error, bn′=bn) of the FBI bit. When n is even,                if bn′=0 then ψ2,(n+1)=0,        otherwise ψ2,(n+1)=0        
When n is odd,                if bn′=0 then ψ2,(n+1)=π/2,        otherwise ψ2,(n+1)=−π/2        
The transmission carrier phase ψ2,(n+1) of the second antenna at the slot (n+1) is finally obtained by the following equation from the provisional carrier phases of the slots n and (n+1).φ2,(n+1)=(ψ2,n+ψ2,(n+1))/2
There can be the case where an error occurs in an FBI bit in the uplink. In this case, since the base station performs transmission with a carrier phase different from that of the control command from the mobile station, an appropriate phase control cannot be performed, thereby increasing an error rate. To solve this problem, the mobile station performs the antenna verification process of estimating a transmission weight (transmission carrier phase) at each slot of the DPCH. An example of the antenna verification is described in, for example, the non-patent document 1, annex A.1, Antenna Verification.
Briefly described below is an example of an antenna verification process. By the following equation, the antenna verification process of the transmission carrier phase of the second antenna is performed. That is,
when n is even,                if 2Σ(1/σ12){√2·Re(γξD2,n,l′ξCP*2,n,l′)        >1n{P(ψ2,n=σ)/P(ψ2,n=0)},        then {ψ1,n′, ψ2,n′}={0, 0}        otherwise {ψ1,n′, ψ2,n′}={0, σ}        
When n is odd,                if −2Σ(1/σ12){√2·Im(γξD2,n,l′ξCP*2,n,l′)        >1n{P(ψ2,n=σ/2)/P(ψ2,n=−π/2)},        then {ψ1,n′, ψ2,n′}={0, −π/2}        otherwise {ψ1,n′, ψ2,n′}={0, σ/2}        
where Σ indicates a sum of l=1 to L, l is an index of path. τDi,n,l′ and ξCP*i,n,l′ are momentary channel estimated values of the DPCH and CPICH of the 1-th path of the n-th slot in the transmission antenna i, respectively, and γ indicates the ratio of the SIR (Signal to Interference Ratio) of the DPCH to the SIR of the CPICH, σ12 indicates the thermal noise and interference power of each path, and P(•) indicates a presumed probability. For example, when it is estimated that the error rate of the FBI bit in the uplink is 4%, and if an FBI bit corresponding to ψ2,n is transmitted, then P (ψ2,n=0)=96%.
When the antenna verification process is to be performed, φ2,(n+1) of the transmission carrier phase of the second antenna of the slot (n+1) is expressed by the following equation.φ2,(n+1)=(ψ2,n′+ψ2,(n+1)′)2
When the antenna verification process is not performed, a mobile station assumes that there is no error in the FBI bit transmitted by the station in the uplink, and performs reception in the downlink.
Generally, when there is an error in an FBI bit in the uplink, the characteristic of the downlink is improved when the above-mentioned antenna verification process is performed.
However, since the antenna verification process is the function of correcting an error in phase control when an FBI bit is erroneous in the uplink, there is a possibility that it is determined there is an error in phase control when there is no error in the FBI bit in the uplink and the transmission in the downlink is performed with an appropriate phase. In this case, although the transmission is performed with an appropriate phase, the receiver receives data with wrong determination information about the phase, thereby increasing the error rate. Therefore, when an FBI bit error rate is low, a higher characteristic can be obtained if the antenna verification process is not performed at all, and the antenna verification process is recommended when the FBI bit error rate is high.
The FBI bit error rate varies depending on the transmission rate of the channel in the uplink, a spreading ratio, a transmission time interval (transmission time interval: hereinafter referred to as a TTI for short), the number of FBI bits, a target error rate, and a slot format. Normally, when the transmission rate is high, the FBI bit error rate is low. When the transmission rate is low, the FBI bit error rate is high.
An FBI bit is mapped over the dedicated physical control channel DPCCH, not over the dedicated physical data channel DPDCH. Therefore, when a comparison is made between a low amplitude ratio of the DPCCH to the DPDCH and a high amplitude ratio thereof, the FBI bit error rate is lower when the amplitude ratio is high. The dedicated physical data channel DPDCH indicates a data channel in a physical layer, and the dedicated physical control channel DPCCH indicates a control channel in a physical layer.
Generally, when a mobile station performs the antenna verification process, the antenna verification process is performed on all channels. When the antenna verification process is not performed, the antenna verification process is not performed on any channel.
There is a technology of stopping the antenna verification process when the status of the transmission line is good (for example, JP2004-179931A; hereinafter referred to as patent document 1).